A light-emitting element using a light emitting material has features of thinness and lightweight, high response speed, low direct-current voltage drive, and the like, and is expected to be applied to a next-generation flat panel display. A light emitting device in which light emitting elements are arranged in a matrix is said to have superiority in wide viewing angle and high visibility as compared with conventional liquid crystal display devices.
The light emission mechanism of a light-emitting element is as follows: electrons injected from a cathode and holes injected from an anode are recombined in the emission center in a light emitting layer to form a molecular exciton by applying a voltage to a pair of electrodes with the light emitting layer interposed therebetween, and energy is released to emit light when the molecular exciton returns to the ground state. An excited singlet state and an excited triplet state are known as an excited state, and it is believed that light can be emitted through either state.
As for the light-emitting element, there are many problems related to materials in improving characteristics thereof. Therefore, improvement of an element structure, development of a material, and the like are conducted in order to overcome these problems.
As an example of a material that is used for a layer containing a light emitting material, a material having a carbazole skeleton (carbazole derivative) excellent in photoconductivity can be cited. Specifically, 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCBP) can be cited (refer to Patent Document 1).
TCBP is proposed as a material for forming a hole transporting layer. However, many of materials having a carbazole skeleton have a larger ionization potential, and a hole injecting property from an electrode is not so good.
On the other hand, as a material that is often used for a hole injecting and a hole transporting material, for example, 4,4′-bis(N-{4-[N,N-bis(3-methylphenyl)amino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD) represented by a structural formula (a) can be cited (refer to Patent Document 2).

DNTPD has smaller ionization potential and superiority in the hole injecting property. In addition, DNTPD has a hole transporting property, and is often used for a hole injecting layer and a hole transporting layer of a light emitting element. However, it cannot be still said that DNTPD has sufficient properties, and development of a material which has better characteristics is required.    [Patent Document 1] Japanese Patent Application No. 3210481    [Patent Document 2] Japanese Patent Application Laid-Open No. H9-301934